2015A&A...577A.127N

The massive and luminous star-forming region W49A is a well-known Galactic candidate to probe the physical conditions and chemistry similar to those expected in external starburst galaxies. We aim to probe the physical and chemical structure of W49A on a spatial scale of ∼0.8pc based on the JCMT Spectral Legacy Survey, which covers the frequency range between 330 and 373GHz. The wide 2x2-arcmin field and the high spectral resolution of the HARP instrument on JCMT provides information on the spatial structure and kinematics of the cloud traced by the observed molecular lines. For species where multiple transitions are available, we estimate excitation temperatures and column densities using a population diagram method that takes beam dilution and optical depth corrections into account. We detected 255 transitions corresponding to 63 species in the 330-373GHz range at the center position of W49A. Excitation conditions can be probed for 14 molecules, including the complex organic molecules CH₃CCH, CH₃CN, and CH₃OH. The chemical composition suggests the importance of shock, photon-dominated region (PDR), and hot core chemistry. Many molecular lines show a significant spatial extent across the maps including CO and its isotopologues, high density tracers (e.g., HCN, HNC, CS, HCO⁺), and tracers of UV irradiation (e.g., CN and C₂H). The spatially extended species reveal a complex velocity-structure of W49A with possible infall and outflow motions. Large variations are seen between the subregions with mostly blue-shifted emission toward the eastern tail, mostly red-shifted emission toward the northern clump, and emission peaking around the expected source velocity toward the southwest clump. A comparison of column density ratios of characteristic species observed toward W49A to Galactic PDRs suggests that while the chemistry toward the W49A center is driven by a combination of UV irradiation and shocks, UV irradiation dominates for the northern clump, eastern tail, and southwest clump regions. A comparison to a starburst galaxy and an active galactic nucleus suggests similar C₂H, CN, and H₂CO abundances (with respect to the dense gas tracer ³⁴CS) between the ∼0.8pc scale probed for W49A and the >1kpc regions in external galaxies with global star formation.